Updated Serbian translation
Updated Galician translations
Completion: simplify add/remove provider
Completion: fix use of G_UNLIKELY
Completion: simplify select_providers()
Completion: little clean-up of populating_done()
Completion: merge cancel_completion() into reset_completion()

And adapt the code to use only reset_completion().
Completion: connect signals with g_signal_connect_object()

The signal handlers are automatically disconnected when the Completion
object is destroyed.